Lithium secondary batteries have rapidly come into wide use in recent years because they are of high voltage, excellent in charge and discharge characteristics, moreover light in weight and small in size, particularly those having a high electromotive force of 4-V class being strongly demanded. As to such lithium secondary batteries, there are known those which use a complex oxide of cobalt or nickel with lithium as the positive electrode active material; but cobalt and nickel are expensive and moreover have the problem of possible exhaustion of resources in the future.
Lithium manganate, which is a complex oxide of manganese and lithium, represented by the chemical formula LiMn2O4 and has a spinel type crystal structure, is useful as the positive electrode active material of 4-V class lithium secondary batteries. Furthermore, since manganese of the raw material is inexpensive and is rich in resources, lithium manganate is promising as a material which can replace lithium cobaltate and lithium nickelate.
Positive electrode active materials are kneaded with various additives and then shaped, or additionally incorporated with a solvent to form a paste and then coated on a substrate. Lithium manganate obtained by the conventional wet process is only of small particle diameters and, even when it is subjected to fireing to effect particle growth, desired large particles cannot be obtained. Resultantly, it shows a low packing density and cannot be filled in a large amount in a fixed volume, so that it cannot give a product of high energy density. It is generally considered that the packing density of powder increases as its particle diameter is increased (that is, its specific surface area is decreased); accordingly lithium manganate with a large particle diameter is greatly demanded.
JP-A-10-194745 discloses a method of increasing the particle diameter of lithium manganate which comprises mixing a manganese oxide and a lithium salt, and subjecting the mixture to primary fireing, then to a treatment for decreasing crystallinity, e.g., mechanical grinding, and further to secondary fireing. In this method, however, since the reactivity of a manganese compound with a lithium compound is poor, a homogeneous composition can be hardly obtained even when fireing is conducted at high temperatures, and lithium manganate having many lattice defects is produced. Moreover, since the product is a non-uniform sintered body obtained by sintering particles, the particle diameter and the particle shape are difficult to control.
JP-A-10-172567 discloses a method which comprises mixing manganese dioxide or a manganese compound with a lithium compound in an aqueous solution, then drying the mixture with a spray drier, granulating the dried product, followed by fireing JP-A-10-297924 discloses a method which comprises synthesizing lithium manganate powder, then densifying and agglomerating the powder, followed by classification and granulation, and then fireing the granulated product. Though these methods give lithium manganate, which makes the base substance, that has few lattice defects and has a uniform composition, difficulties remain unsolved in that the particle diameter and particle shape are difficult to control and that the particles ultimately obtained are non-uniform sintered bodies.